Production of uranium sulphate



PRODUCTION OF URANIUM SULPHATE Application October 20, 1044, Serial No. 559,662

6 Claims. c1. 23-145 No Drawing.

This invention relates to the recovery of uranium from mixtures containing lower uranium oxides. It is particularly concerned with the treatment of m'mtures in which substantial proportions of the uranium are present as free metal or as uranium tritaoctaoxide (U308) or dioxide (U02).

In the past it has been proposed to treat hexavalent uranium ores such as carnotite with sulfuric acid to convert the uranium to uranyl sulfate. However the treatment of tetravalent forms of uranium in the same way is relatively ineffective for dissolving the uranium because these forms of uranium are soluble only with difliculty.

It is an object of the invention to convert oxides of uranium of valence less than six to water-soluble form. A further object is the conversion of tetravalent uranium oxides to soluble form without the use of acid-resistant high temperature converters and other relatively expensive equipment. A still further object of the invention is the conversion of lower uranium oxides to uranium sulfates. Yet further objects will become apparent from the following general description of the invention and the detailed illustrations of its application.

In accordance with the present invention an oxide of uranium of valence less than six is reacted with dilute sulfuric acid in the presence of a fluoride. I have found that the presence of a fluoride, even in exceedingly small amounts, greatly facilitates the conversion of these lower oxides of uranium to readily soluble form.

For the treatment of mixtures containing metallic ura nium an oxidizing agent may be employed to oxidize the metal at the same time that it is being dissolved. In the case of uranium in tetravalent form no oxidizing agent is necessary to effect its solution in the dilute sulfuric acid as long as a fluoride is present. However, in many cases, it is desirable to convert the dissolved tetravalent uranium to the hexavalent form since in general tetravalent uranium has a lower solubility than hexavalent uranium and when present in high concentration tends to precipitate as basic sulfate from sulfuric acid solution upon partial neutralization or excessive dilution. Accordingly, an oxidizing agent may be introduced into the solution either during the dissolving process or after its completion to efiect this conversion.

The process of the invention is especially suitable for the treatment of slags, or other scrap materials, designated generally dross herein, or air-roasted products thereof, obtained in the manufacture of metallic uranium by the tetrafluoride process, or of similar residues obtained by other processes.

In the tetrafluoride process uranium tetrafluoride is reacted with an alkali-metal or alkaline earth metal (including magnesium) or with beryllium or aluminum in a reaction vessel, which may be lined with a refractory material such as lime, and there is produced, in addition to a main body of metallic uranium, a slag comprising fluorides and oxides of the metals present and also quantities of unreacted liberating metal and metallic uranium which has failed to cohere with the body of metallic 2,741,541 Patented Apr. 10, 1956 uranium produced as the primary product. Moreover in remelting and in working the metal, scraps may be produced of insuflicient purity to permit their being worked up into finished metal products.

The uranium present in such dross is normally present as the free metal together with relatively small amounts of tetravalent uranium compounds. To render the metal more readily soluble, I subject it to a preliminary roast in air at a temperature between 300 and 1000 C. The preliminary air roast converts metallic uranium to lower oxides. From 30% to 90% or more of the uranium may be converted to U303 and most of the remainder to U02. A part of the U303 may react with alkalimctal or alkaline earth metal oxides when these materials are present, to form uranates, but in many cases a considerable proportion, at least, remains in the form of lower oxides.

For converting such products to soluble form, I may employ a dilute sulfuric acid solution containing 0.5% to 50% H2804 by weight. If the mixture to be treated is initially free of fluorides, a metal fluoride, such as magnesium, calcium or sodium fluoride, or hydrogen fluoride may be used. An amount of fluoride providing a fluorine content of the order of 5% of the uranium present is ample. The mixture to be dissolved is mixed with the dilute fluoride-containing sulfuric acid solution and heated at an elevated temperature, preferably between about C. and the boiling point of the solution. This treatment forms a mixture of suifates of tetravalent and hexavalent uranium. If free metal is present, an oxidizing agent more active than H2804 at the dilution employed, for example, air hydrogen peroxide, nitric acid, or a permanganate or chromate is added to oxidize the free metal. If it is desired to convert uranium to hexavalent form (as U02S04), this may be done byadding additional quantities of such an oxidizing agent.

It is not necessary that the quantity of sulfuric acid solution present during heating be suflicient to dissolve the uranium compounds completely. The reaction proceeds even though the soluble compound is formed in quantities exceeding its solubility. The product can then be dissolved completely merely by adding more water or sulfuric acid solution.

Hexavalent uranium may he recovered from the sulfuric acid solution by conventional recovery processes or, more advantageously,v by the peroxide recovery method of Louis Spiegler, U. S. patent application Serial Number 559,665, filcdOctober 20, 1944, entitled Method of Isolating Uranium.

The following examples illustrate the invention. Proportions are by weight where not otherwise indicated.

Example 112 An 82-gram sample of a slag which was obtained from the production of metallic uranium by the tetrafluoride process and which contained 30 /2 of uranium, 60% of which was present as hexavalent uranium, and the balance present as tetravalent and lower-valent uranium, and a substantial proportion of calcium and magnesium fluorides Was heated with 250 grams of aqueous 20% sulfuric acid at a temperature of 85 to C. for one hour. Since part of the acid was used up in neutralizing alkaline material in the slag, the effective concentration of H2804 in the solution was 10.7%. The uranium'was completely dissolved at the end of the hour.

Example 1 b A 29.4-grarn sample of an oxide product containing 85% uranium (U308) (obtained by roasting in air contaminated uranium metal) was heated in 228 grams of aqueous 12% sulfuric acid at a temperature of 85 to 95 C. for 1% hours. The product was practically unattacked by this treatment. A Z-gram portion of calcium Example? "A uranium 'dross-left in'the crucible when crude ,urani" urn was remelted and recast was roastedjin'air'at 800 C. ,and milledto pass through 211200 mesh screen. 'Five samples ofthe roasted dross, containing 79% uranium and .3 /z%,fluorine, were mixed with sulfuric acid as follows:

f(a) A lfifl gramsample, of theroasted'dross was-mixed with '372 gramsof aqueous-47% sulfuric acid'and heated 'toatemperatureof 95 to 100 C.;

(b). A lgramsample of the roaste'ddross was mixed withl5 grams ofiaqueous 34% sulfuric'acid and heated 1095 to l00 (3.;

"(0') AJIOO-gram sample of the roasted dross was mixed "with'372'grams ofaqueous 47% sulfuric acid and heated to 85 to'90 C.;

(d) A'ltlO-gramsample of the'roasteddross was mixeid with 515 grams'of aqueous 34% sulfuric acid and heated 'to .85 to-90 'C.-; a

'(e) 'Al00-gram'sample of the roasted dross was mixed -with 372jgrams of aqueous 47% sulfuric acid andheated toll7" to 118 C. The quantity of ,alkalinecompounds in 'thegroasted dross was such that after reaction'thereof with sulfuric acid the HzSOeconcentra'tion at the start .of heating was 39% for samples (a), '(c) and (e) and 27%:for'samp1es (bland (d). Thefive samples were maintainedat'the specified temperatures between one'and one and one-half hours. The volumes :were, then made up toj500 to 550 cc. by addingwater and the mixtures were cooled to"40 C. 'An aqueous solution of"28% hydrogen peroxide'(approximately 16 cc.) was added .to each solution, and the solution wasstirred 1'5 to minutes, then heated to the boiling point to decomposeanyuraniumgperoxide formed andto produce soluble uranyl sulfate. Solid matter was then separated from the solution .byffiltra- ,tion andith'e'filter cakes were testedfor unanium. The uranium "detected in'this manner ineach of the samples was as follows:

.Sample'(a);none Sample (b)--uone Sample (c)0.1% of the uranium in the original sample Sample d)0. 05 %-of the uranium in the original sample Sample- (-e)0;06% -of the uranium in the original sample -Sincethe alkalineearth metal fluorides are onlysparingly.,soluble.:in .dilute H2804 or-neutral solution, the major .prop0r.tion.of. the fluorides present in-slags ordross of the types: considered above remain undissolved and may be .Io. .one :ot .the .mixturesonegart of sodium fluoride i it Nah (added. ,The ,aqueous phase of this mixture immediately aexhibited ;a greenish :cast. Both mixtures -:were:hldzatl hoilingztemperature;.for /z hou13and water was added to maintain the volumes constant. The two "products-we're "filtered and the "filter-residue was -waslred, dried and weighed. The residue of the portion to which no Na was added amounted to 4 /2 parts. The residue 5 of the portion to which NaF was added amounted to nil, showing that in this case solution of the uranium dioxide was complete.

Exampie'4 f27rgrams='ofifinelydivided uraniurn'dioxide: (U02) and 10 grams of NaF were added to 'l'40sgramso'f a H2804 solution, and the mixture was heated at'80-'-'85 C. for threehourspwatetbeing-'added .to maintain a=constant volume. The brown oxide-wentinto solution and green crystals precipitated. The ,mixture was diluted with water to a total volume of'onelit'er. "The solution was arich green and the green crystalline precipitate was completely dissolved. The solution was filtered and the filter residue was weighed. It consisted'of 0.2 gram of brown oxide.

'JInU. S:patent application Serial Number 559,66t5,"'entitled Recovery of Uranium from Waste" Metal Products and filed October 20, 1944, by Louis Spiegler, there is described aproeessfor recovery of uranium from "mixtures containing uranium and a more active metal in'the form of oxides and '{fluorides or other compounds -con taining ithese elements. The process involves roasting thematerial-with lime in air and treating 'the-product'at elevated temperatures with concentrated sulfuric acid. The productof' this sulfuric acid treatment is introduced .intowater to dissolve "the soluble components. This-step produces a relatively "dilute sulfuric acid solution con taininguranyl-sulfateand more active metal sulfateye'g. -magnesium sulfate.

The -process-of the present invention may be combined with the-process described in the 'Spicglcrapplication by 'ernployingthe aqueous sulfuric acid solution, obtained'as just-described, for dissolving 'tetravalent uranium in ac :cordance -with the method of the present application. The fiuorinenecessary to-eltect solutionof the tetravalent uranium may be presentin the sulfuric acid solution,*-out fluorine in the formof-a fluoride may be added with the 'tetravalent uranium. The resulting aqueous sulfuric-acid solution of uranium sulfates may then be treatedwith an oxidizing agent to-convert tetravalent uranium 'to'hexa- -valent-forrnand the uranium may be recovered "from the solution in anyconvenient manner.

In "thespecification'and claims the expression oxide "oftetravalent-uranium is intended to include the oxide, U308 "01' UOz'lUos 'in-which part of-the uranium --is tetravtilent-and ,p'art; isjhexavalent, "and the dioxide, U02,

'inwhioh' allot-the uranium is tetravalent.

The term alkaline earth metal 'is used herein and'in ,the "appended claims' to designate the elements calcium, magnesium, barium and'strontium.

'{It will be understood that I intend to include variations i and modifications of the invention and that the preceding examples -are illustrations ,only and in no 'wise jtobe construed aslirriitations upon the invention, the scope-of which is defined in the appended claims, wherein 'I claim: i

l. The method 'of'dissolving an oxide of uranium of valenceless'than'six, which comprises treating-the oxide with dilute aqueoussulfuric acid containing a fluoride.

ZJThemethod of dissolving an "oxide .oftetravalent uranium, which'coutprisesheatingthe oxide with'a'queous 0.5% to-'"50% sulfuric'acid solution containing'a fluoride.

3. The method of dissolving metallicuranium, which comprises roastingithe metallic uranium in airtoconvertit ito .an oxide of tetravalent uranium. andheating ,the oxide withaqueous 0.5% .to

tion containinga fluoride. V

.4. The methodofrecovering guranium from a uranium dross,coutaiuiugealkaline :earth zmetal and; fluorine, which comprises;,heating;;the dross rwithaque'ous, 0.5 .to ;'50.% 'g'Sillflll'iC acid ..-solution rat 1 :a. temperature between :about sulfuric acid .solu- 80 C. and the boiling point of the solution, adding an active oxidizing agent to convert tetravalent uranium t hexavalent form, and separating the uranium solution from undissolved alkaline earth metal fluoride.

5. The method of converting metallic uranium to uranyl sulfate, which comprises roasting the metal in air to convert it to an oxide of tetravalent uranium, heating the oxide with aqueous to 50% sulfuric acid containing a fluoride, thereby converting the oxide to tetravalent and hexavalent uranium sulfates, and adding an oxidizing agent more active than sulfuric acid to the mixture to convert the tetravalent uranium sulfate to uranyl sulfate.

6. The method of converting metallic uranium to uranyl sulfate, which comprises roasting the metal in air to convert it to an oxide of tetravalent uranium, heating the oxide with aqueous 0.5% to 50% sulfuric acid con- 6 taining 21 fluoride at a temperature between about C. and the boiling point of the solution, thereby converting the oxide to tetravalent and hexavalent uranium sulfates, and adding an oxidizing agent more active than sulfuric acid to the mixture to convert the tetravalent uranium sulfate to uranyl sulfate.

Mellor: Inorganic and Theoretical Chemistry, vol. 12, p. 49, Longmans, Green & Co., London (1932). 

1. THE METHOD OF DISSOLVING AN OXIDE OF URANIUM OF VALENCE LESS THAN SIX, WHICH COMPRISES TREATING THE OXIDE WITH DILUTE AQUEOUS SULFURIC ACID CONTAINING A FLUORIDE. 